The present invention relates to a method and an apparatus for treating absorption slurry containing fly ash. More specifically, it relates to a method and an apparatus for separating fly ash which is present in an absorbent slurry out of a desulfurization apparatus for exhaust gases by a wet lime method.
When an exhaust gas from coal burning is desulfurized according to a wet lime method, an absorbent solution absorbs SO.sub.x present in the exhaust gas. Also, an exhaust gas often includes HF as a harmful component. When such exhaust gas is treated with CaCO.sub.3 as an SO.sub.2 absorbent in a wet type exhaust gas absorption tower, the following reactions occur: EQU CaCO.sub.3 +SO.sub.2 .fwdarw.CaSO.sub.3 +CO.sub.2, (1) EQU CaCO.sub.3 +2HF.fwdarw.CaF.sub.2 +CO.sub.2 +H.sub.2 O. (2)
However, the exhaust gas contains fly ash as well as SO.sub.x and HF, and this fly ash also mixes into the absorbent solution. If the absorbent solution, as a result, contains a large amount of fly ash, the aluminum (Al) component in the fly ash dissolves and produces fluorine compounds of aluminum (called AlF.sub.x hereinafter) by reacting with HF present in the gas. The AlF.sub.x thus produced is known to inhibit the dissolution effect of limestone (CaCO.sub.3) (Japanese Patent Provisional Publication No. 16702/1970 (or Showa55-16702)).
To cope with this, it is effective to prevent fly ash, a source of aluminum, from mixing into the absorbent solution. It is thus common to dispose a dust separator, such as an electrostatic precipitator, in an upstream position from the exhaust gas desulfurization apparatus. However, it is difficult to remove fly ash completely, and the mixing of fly ash into the absorbent solution is unavoidable in reality.
Also, the fly ash which mixes into the absorbent solution is fine particles whose average diameter is 5 .mu.m or less, and since the speed of precipitation is slow, removal by precipitation separation requires a thickener with an extremely large sectional area. The separation is difficult with a thickener only.
Furthermore, if the absorbent solution contains a large amount of fly ash, it has been a disadvantageous problem that collected gypsum will also contain fly ash and the recovery of highly pure gypsum is impossible.